1. Field of the Invention
The present invention relates to automatic refractive power measurement for an eyeglasses lens and a contact lens.
2. Related Background Art
In a prior art automatic lensmeter or conventional lensmeter, a lens to be tested is positioned by referencing an optical eccentricity, that is, a prism value. When the eccentricity is displayed on a screen, it is displayed by referencing the eccentricity as if it were a display for a projection type lensmeter. The sign for the completion of positioning, the positioning for the refractive power measurement and the positioning for marking the center of the lens are effected by referencing the prism value and only the results thereof are indicated to the operator. Even in a best case, the refractive power is classified to one of several levels and the reference of the discrimination is changed in accordance with the level.
In the prior art methods and apparatus, both the positioning for measurement and the positioning for marking reference the prism value.
However, the marking refers to an actual distance due to the restriction by a mechanical precision and the restriction by a machining precision. Accordingly, when the degree of the lens is low, the positioning precision is low, and when the degree is high, the positioning precision is high and the positioning is difficult to attain. That is, the positioning is easy or difficult depending on the refractive power of the lens to be tested.
For example, when the refractive power of the lens is high, the positioning is very hard to attain and a target moves off the screen by slightly moving the lens. Thus, it is difficult to position the lens while watching the display.
When a mixed astigmatism lens (which has a larger cylindrics than a spherics with opposite polarities, such as a lens having a spherics 3DP and a cylindrics -4DP) is measured, the target may move vertically while the lens is moved horizontally. In such a case, it is very difficult to position the lens while watching the display.
In a prior art apparatus which produces a sign for the completion of positioning, the completion is determined by the prism value. As a result, the positioning for marking must be effected with a higher precision than that actually required. This is a heavy burden to the operator in the lens positioning operation.
A progressive focal length lens has recently been taking the place of a bi-focal length lens as an eyeglasses lens for an aged person. However, the lensmeter does not necessarily comply therewith.
Unlike a conventional eyeglasses lens, the progressive focal length lens has a specially curved plane other than a spherical plane or a cylindrical plane. As shown in FIG. 11, a refractive power is substantially constant in a distance area 211 which occupies an upper half of the lens but it varies from point to point in a close area 212. The close area 212 consists of an optically effective, narrow band-shaped area called a progressive area in which the refractive power progressively increases from the top down as viewed in FIG. 11 and a non-effective area (non-optical area) 213 surrounding the progressive area. The difference therebetween cannot be distinguished at a glance. Accordingly, the refractive power in the distance area can be relatively easily measured but the refractive power in the close area 212 is not easy to measure. The lens includes a wide non-effective area in which focusing is not attained by a manual lensmeter. Even by an automatic lensmeter, the measurement is not attained unless a measuring light is precisely directed to the progressive area because the diameters of the progressive area and the measuring light beam are of the same order.
As described above, the progressive focal length lens has the specially curved plane and has no uniform refractive power. Accordingly, by the prior art lens alignment which references the optical eccentricity, it is very difficult to specify the position. Actually, it is almost impossible.